In the production of oil from subterranean oil reservoirs by various flooding techniques, especially waterflooding, it has become a common expedient to add various polymeric thickening agents to the water in order to increase its viscosity to a point where it approaches that of the crude oil which is to be displaced so as to improve the displacement of the oil from the reservoir. Use of polymers for this purpose is often stated to be for mobility control.
Another problem which arises in the various flooding processes is that different strata or zones in the reservoir often possess different permeabilities so that displacing fluids enter the high permeability of "thief" zones in preference to zones of lower permeability where significant quantities of oil may be left unless measures are taken to plug the high permeability zones wholly or partly and so divert the displacing fluid into the low permeability zones. Mechanical isolation of the thief zones has been tried but vertical communication among reservoir strata often renders such measures ineffective. Physical plugging of the high permeability zones by cements and solid slurries has also been attempted with varying degrees of success but here, the most serious drawback is the possibility of permanently closing still productive horizons.
From these early experiences, the desirability or designing a viscous slug capable of sealing off the most permeable layers, of slowly moving from injector to producer, and of diverting the trailing floodwater to the underswept, tighter regions of the reservoir, became evident. This led to the use of oil/water emulsions, gels and polymers for controlling the permeability of the formations in a process frequently referred to a "profile control", a reference to control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed have included oil/water emulsions, gels, e.g. lignosulfate gels and polymers, with polymers being the most extensively applied in recent years.
Among the polymers so far examined for improving waterflood conformance are polyacrylamides (note J. C. Mack, "Process Technology Improves Oil Recovery," SPE 7179, SPE Rocky Mountain Regional Meeting, May 17-19, 1978, Cody, Wyo.; W. G. Routson, M. Neale, and J. R. Penton, "A New Blocking Agent for Water Channeling," SPE 3992, 47th Ann. Fall Meeting of SPE-AIMR, Oct. 8-11, 1972, San Antonio; D. Sparlin, "An Evaluation of Polyacrylamides for Reducing Water Production," J. Pet. Tech., 906-914, August, 1976; and G. P. Willhite and D. S. Jordan, "Alteration of Permeability in Porous Rocks with Gelled Polymers," 1981 ACS Meeting, Aug. 23-28, New York, Polymers Preprints), polysaccharides, carboxymethylcellulose (note R. W. Farley, J. F. Ellebracht, and R. H. Friedman, "Field Test of Self-Conforming Oil Recovery Fluid," SPE 5553, 50th Ann. Fall Meeting of SPE-AIME, Sept. 28--Oct. 1, 1975, Dallas), furfural-alcohol and acrylic/epoxy resins (note R. H. Knapp, M. E. Welbourn, "An Acrylic/Epoxy Emulsion Gel System for Formation Plugging: Laboratory Development and Field Testing for Steam Thief Zone Plugging," SPE 7083, Symposium on Improved Oil Recovery, April 16-19, 1978, Tulsa; and P. H. Hess. C. O. Clark, C. A. Haskin and T. R. Hall, "Chemical Method for Formation Plugging," J. Pet. Tech., 559-564, May, 1971), and polyisocyanurate (note C. T. Presley, P. A. Argabright, R. E. Smith, and B. L. Phillips, "A New Approach to Permeability Reduction," SPE 4743, Symposium on Improved Oil Recovery, April 22-24, 1974, Tulsa). A major part of this work has been conducted with the polyacrylamides.
Polyacrylamides have been used both in their normal, noncrosslinked form as well as in the form of cross-linked metal complexes. In either form, the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection and sensitivity to reservoir brines. To overcome these problems and to achieve deeper penetration into the reservoir, dilute solutions of these polymers have sometimes been injected first and then complexed in situ. For example, in one such process, three sequential injection steps are employed: cationic polyacrylamides are injected first for strong adsorption and anchoring onto the generally anionic sites of the reservoir rock surfaces, followed by chelation with aluminum ions provided by aluminum citrate or with chromium ions generated by the in situ reduction of dichromate ions and finally, anionic polyacrylamides are injected for the formation of the desired cationic polymer-metal ion-anionic polymer complexes (J. E. Hassert, and P. D. Flemming, III, "Gelled Polymer Technology for Control of Water in Injection and Production Wells," 3rd Conference on Tertiary Oil Recovery, U. of Kansas, Lawrence, 1979).
Another group of polymeric thickeners which has received considerable attention for use in waterflooding is the polysaccharides, particularly those produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose mobility control by the use of polysaccharides in the presence of polyvalent metal ion crosslinking agents. U.S. Pat. No. 3,810,882 discloses the possibility of using certain reducible complex metal ions as cross-linking agents for polysaccharides and U.S. Pat. Nos. 4,078,607 and 4,104,193 describe a method for improving the efficiency of waterflooding operations by a particular polysaccharide prehydration technique.
U.S. Pat. No. 3,908,760 describes a polymer waterflooding process in which a gelled, water-soluble Xanthomonas polysaccharide is injected into a stratified reservoir to form a slug, band or front of gel extending vertically across both high permeability and low permeability strata. This patent also suggests the use of complexed polysaccharides to block natural or man made fractures in formations. The use of polyvalent metal ions for cross-linking polysaccharides and other polymers including polyacrylamides which are to be used for permeability control is described in U.S. Pat. Nos. 4,009,755; 4,069,869 and 4,413,680.
One problem which has continually attended the use of polymeric mobility and profile control agents is that thickened aqueous solutions, e.g. polysaccharide solutions, may be more difficult to inject into the reservoir than less viscous solutions. Also, the shear conditions encountered during injection may degrade the polymer and reduce its effect when it enters the reservoir. To overcome the injectivity problem, U.S. Pat. No. 3,208,518 proposes the use of polymer solutions of controlled pH which undergo a delayed increase in viscosity after the solution enters the formation and the pH changes by neutralization of acidic or basic constituents in the solution by materials present in the reservoir.
In general, there are two ways to deliver polymer gels into the formation. The first method is to inject gelled polymer into the formation. This is the so-called surface gelation method. The advantage of this method is that the polymer will enter the loose zone in preference to the tight zone because of the high viscosity of gelled polymer. The other advantage is that gelation is ensured because the gel is prepared on the surface. The disadvantage of this method is that the polymer gel will probably not penetrate far enough to block a high pore volume of the designated zone at low pumping pressures and low pumping rates, especially when the pressure drop occurs rapidly within a small radius of the injection wellbore. At high pumping pressures and flow rates, there are increased risks of fracturing the reservoir and degrading the gel structure by high shear forces, as previously mentioned.
The second method is the so-called in situ gelation method, in which separate slugs of polymer, one containing an inactive crosslinker (such as dichromate) and the other activator (reducing agents such as thiourea and bisulfite), are injected sequentially into the reservoir. Gelation occurs when the two parts meet in the reservoir. With this method, shear degradation is reduced and the penetration of polymer is improved because of the lower viscosity of the ungelled polymer. However, because of its low viscosity, the uncrosslinked polymer slugs can also enter the tight zone and cause its blockage which defeats the purpose of profile control. Another disadvantage of this method is that there is no guarantee that the two slugs of treatments will be placed in the same area and mixed well enough to form a strong gel.
It would therefore be desirable to devise a method for delivering the gelled polymer into the formation in a manner which ensured the formation of a strong gel when the polymer was correctly placed in a large volume of the formation and which avoids the problems associated with high injection pressures, pumping rates and shear forces.